Combined electrochemical and spectroscopic investigations of carbonate-mediated water oxidation to peroxide

Summary The development of electrosynthetic technologies for H2O2 production is appealing from a sustainability perspective. The use of carbonate species as mediators in water oxidation to peroxide has emerged as a viable route to do so but still many questions remain about the mechanism that must be addressed. To this end, this work combines electrochemical and spectroscopic methods to investigate reaction pathways and factors influencing the efficiency of this reaction. Our results indicate that CO32− is the key species that undergoes electrochemical oxidation, prior to reacting with water away from the catalyst. Through spectroelectrochemical experiments, we noted that CO32− depletion is a factor that limits the selectivity of the process. In turn, we showed how the application of pulsed electrolysis can augment this, with an initial set of optimized parameters increasing the selectivity from 20% to 27%. In all, this work helps pave the way for future development of practical H2O2 electrosynthetic systems.


INTRODUCTION
Hydrogen peroxide is a widely used oxidant, which has the highest oxygen content by weight among the peroxides and is effective in the whole pH range.Employing H 2 O 2 in a reaction generally results in oxygen and water as the main byproducts, rendering it a potentially green oxidant. 1 H 2 O 2 is conventionally produced by the anthraquinone process which possesses limitations of safety and a substantial carbon footprint. 2In particular, this process uses H 2 produced from the steam reforming of methane, a particularly CO 2 -emissive process. 1 An alternate route to H 2 O 2 production is through electrocatalysis. 3The appeal to electrocatalytic H 2 O 2 production is that the process can ideally be powered by renewable electricity at a variety of scales.Furthermore, electrochemical systems are inherently modular as they do not adhere to the same scaling relations as thermochemical processes and thus, electrocatalytic peroxide production can be carried out in a distributed fashion, producing it at the point of use.This is greatly beneficial for applications like water purification in remote communities. 4][7][8] To this end, there are two main catalytic routes: anodic 2-electron water oxidation reaction 9 (2e À WOR, Equation 1) or cathodic oxygen reduction reaction 10 (2e À ORR, Equation 2), which may be performed within a single electrolyzer in parallel.A significant bottleneck lies within the anodic side, predominantly arising from competition with the 4e -WOR (Equation 3) (Figure 1A).
2H 2 O 4 H 2 O 2 + 2H + + 2e À E 0 = 1:76 V vs: RHE (Equation 1) O 2 + 4H + + 4e À 4 2H 2 O E 0 = 1:23 V vs: RHE (Equation 3) Note, the reversible hydrogen electrode (RHE) is used here which is a pH-dependent scale.The product of the reactions is placed on the right-hand side.2][13] However, the high overpotential needed to run 2e À WOR limits the initial electrocatalyst selection pool to the materials stable under harsh oxidative conditions.5][16][17][18][19] Beyond direct water oxidation, the use of carbonate/bicarbonate species as a mediator to promote 2e À WOR has recently been investigated (Figure 1B). 20,21This strategy is promising as its high selectivity can be attained through the selective reaction of oxidized carbonate species with water to yield peroxide, though the exact identity of such reactive intermediates and the mechanism for the overall reaction is not yet established.
Peroxymonocarbonate (HCO 4 À ; Equations 4 and 5) ; Equations 6 and 7) [25][26][27] have been noted as possible intermediates.[32] 4) 6) 7) 8) Against this backdrop, we aimed to carry out an investigative work incorporating electrocatalytic and spectroscopic methods to shed light on this process.Using SnO 2 as a model catalyst, we first probe the role of (bi)carbonate and CO 2 in mediating 2e À WOR.In a complementary direction, we carry out operando (i.e., as the reaction is occurring) spectroscopic infrared and Raman experiments that point to the existence of HCO 4 2À species during the reaction process.We observe that a likely limitation of this system is the depletion of near-surface CO 3

2À
and subsequently implement pulsed electrocatalysis as a proof-of-concept technique that is used to augment selectivity.In all, the insights derived stand to aid the advent of electrochemical H 2 O 2 synthesis through the elucidation of key mechanistic aspects underpinning the process.

RESULTS AND DISCUSSION
Our first endeavor entailed establishing a functional 2e À WOR system.We used a carbon paper electrode as a reference and SnO 2 nanoparticles as a catalyst (Figure S2).Using 2 M KCO 3 as a starting point, we measured the Faradaic efficiency (FE) and partial current density for H 2 O 2 (I H2O2 ) as a reference point.While this species exists as HO 2 À above pH 11.6, we only use the term H 2 O 2 for simplicity.In comparing to the bare carbon paper, we noted that the presence of SnO 2 increased both the FE to a maximum of 20% (Figure 2A) and I H2O2 up to 126 mA/cm 2 (Figure 2B).This indicates that SnO 2 does not only suppress the 4e À WOR but also actively promotes H 2 O 2 production through facilitating CO 3
The potential-dependent FE trends we observe in this work parallel those previously noted for various metal oxides, in which the FE increases with applied potential, then starts to decline. 12,13,22The current density is also dependent on CO 3 2À concentration, increasing until 0.5 M, and plateauing afterward (Figure S3).The production of H 2 O 2 was relatively constant with time, decreasing by approx.5% over the course of 3.3 h (Figure S6) and no obvious changes to catalyst morphology were noted (Figure S7).However, we did see slight changes in the catalyst's surface electronic structure after such measurements from X-ray photoelectron spectroscopy measurements (Figure S8).A red slight shift (approximately 0.2 eV) in the Sn 3d bands is tentatively attributed to restructuring that may be occurring at the catalyst surface throughout the extended catalytic process.
We then tested the role of CO 2 and HCO 3 À in mediated H 2 O 2 production.If the 2 M CO 3 2À electrolyte was saturated with CO 2 instead of N 2 , the FE and I H2O2 both decreased (Figures 2C and 2D).This likely stems from CO 2 functioning as a spectator species and hindering the diffusion and oxidation of CO 3 2À en route to H 2 O 2 production.The pH of the solution also drops in this case to a value of approximately 11 so the effects here may be compounded.If 2 M HCO 3 À is used instead, both the FE and I H2O2 become significantly lower.This may indicate that HCO 3 À is less effective at mediating the 2e À WOR or that it is not active at all.Alternatively, the diminished production that we see stems from CO 3 2À formed in the electrolyte as the pH near the electrode surface drops from H + -producing reactions like the 4e À WOR.Again, the pH of this solution is lower (approx.8.6) so pH effects may also play a role (explored in the following section).Interestingly, this is not always the case in the literature as some reports show higher efficiency when HCO 3 À is used instead 3 and while we do not have a definitive answer to explain these discrepancies, we hope that this work may help close key mechanistic knowledge gaps en route to fully understanding this reaction system and being able to do so.
To deconvolute effects of catalyst, pH, and anions, we performed a series of systematic studies.First, the effects of catalyst surface were probed.We tested the electrochemical system's performance when SnO 2 catalysts were replaced either by a bare carbon paper electrode or by equivalent loadings of TiO 2 or WO 3 particles under otherwise identical conditions (2M M CO 3 1À at 3 V vs. RHE).We observed that the SnO 2 electrode substantially outperforms the rest of the electrodes in terms of FE (Figure 3A) and this is tentatively attributed to the CO 3 2À oxidation being more rapid on SnO 2 surfaces.This notion is also supported by an increase in partial current density with SnO 2 (Figure 3B).In contrast, WO 3 -loaded electrodes feature lower FE but comparable partial current densities at the same potential and this is attributed to their enhanced activity for the 4e -WOR.As we previously observed that CO 2 and HCO 3 À led to a diminished performance, though the pH was different in the bulk electrolyte, we sought to systematically change the electrolyte pH to measure the resultant system's performance.At 3 V vs. RHE, we increased the pH with the addition of KOH, while keeping 2 M CO 3 2À in the electrolyte and SnO 2 as the catalyst (Figures 3C and 3D).We observed that this decreased the FE for peroxide production and this was attributed to an increase in 4e -WOR activity.Increasing the pH to 12.9 did, however, increase the peroxide partial current density.This signifies that the 2e -WOR is maximized around pH 12.9 but the selectivity decreases due to competing reactions.Likewise, if the pH was decreased, in this case by adjusting the CO 3 2À /HCO 3 À ratio, then the performance decreased once again in terms of FE and partial current density.We also ruled out the effects of other ions as mediators for the 2e -WOR.With a SnO 2 electrode and 2 M concentrations of KCl, KNO 3 , K 2 SO 4 , or KOH as the bulk electrolyte, only minimal activity was observed for peroxide production in terms of FE or partial current density (Figures 3E and 3F).If, for example, we adjusted the pH of the KCl or K 2 SO 4 electrolyte to 12, the FE remained at 1.1% and 1.6%, respectively, and the partial current densities only reached 1.6 and 2.1 mA/cm 2 .Any peroxide production in this case likely proceeds via a non-mediated pathway.We next turned to vibrational spectroscopy to probe the identity of possible reaction intermediates and active species in the carbonatemediated 2e À WOR.Our initial efforts centered on infrared (IR) spectroscopy, applied under reaction conditions in an external reflection configuration (Figure S9).We noticed that the majority of the spectrum was dominated by the relative changes of the HCO 3 triggered by under oxidizing potentials that generate H + and lower the pH at the catalyst surface (Figure S10).As such, the HCO 3 À band at 1,616 cm À1 grew, indicating the increase of this species while a negative CO 3 2À band 1,375 cm À1 also increased in magnitude, indicating the relative diminishment of this molecule (Figure 4A).However, we did see a positive absorption band (indication the formation of a species) at 1,289 cm À1 .Candidate species in the carbonate-mediated peroxide production mechanism (HCO 4 À and C 2 O 6

2À
4][35] To this end, we employed the isotope labeling technique to gain further insights into its origin.Using 13 C-labeled CO 3 2À led to a 12 cm À1 red-shift in this band to 1,277 cm À1 , indicating that this species likely originated from CO 3 2À oxidation and vibrational mode featured the C-atom from CO 3 2À (Figure 4B).The redshift was less than that expected for a simple bimolecular C=O stretch and thus this mode may involve multiple atoms.We next probed the temporal evolution of the aforementioned species as the potential was stepped from open circuit to catalytic (3 V vs. RHE).Through acquiring a spectrum every 30 s and integrating the band area, we noted how the negative/positive CO 3 2À /HCO 3 À bands growth mirrored each other.This is reasonable as this mainly originates from their direct interconversion.When the potential was turned off (system reverted to open circuit), the magnitude of both bands began to slowly decrease.In contrast to this, the 1,289 cm À1 band rose to its maximum at 1 min and decreased afterward, even as the potential remained at 3 V.This led us to believe that this species was rapidly formed, then its quantity decreased, possibly it reacted with H 2 O and additional CO , this measurement probed not only the surface but also within 1-2 mm away, rendering it difficult to directly correlate the bands to what is present directly at the electrode surface.
To this end, a subsequent question arose in determining whether this species was a surface-bound intermediate or a solution-based mediator.To address this challenge, we turned to surface-enhanced Raman spectroscopy (SERS) (Figure S13).Because of the surface-sensitive nature of the SERS technique, we hypothesized that surface-bound intermediates would be detected with SERS and IR measurements while near-surface species would only be detected with IR experiments.The catalyst placed a role in the system performance as it affected the rate of carbonate oxidation (A, B).The pH was also critical to peroxide production and optimal around 12 (C, D).Finally, the role of other anions as mediators for peroxide production was ruled out as only CO 3 2À ions gave rise to substantial peroxide generation (E and F).Data are represented as mean G standard deviation.
We fabricated SERS-active electrodes through a simple electrodeposition and galvanic exchange approach. 36The resulting catalysts consisted of an Au underlayer with a SnO x coating that mimics the SnO 2 catalysts used as previously described (Figure S14).As SERS selectively provides information on chemical species within a few nm of the surface, this would reveal if any substantial quantities of surface-bound species would be built up under reaction conditions.However, upon the application of successively oxidizing potentials, the main species detected were CO 3 2À , and HCO 3 À , present near the surface (Figures 4D and S14).CO 3 2À decreased as it was depleted via its oxidation and from the interconversion to HCO 3 À as the pH decreased.We did not see evidence of surface-bound species of HCO 4 , which would have expected to yield strong bands around 900 and 700 cm À1 even as we continually observed (bi)carbonate species in the spectra. 34,35,37,38e took this as evidence for carbonate-derived species rapidly diffusing away from the surface and reacting in the bulk of the solution to yield hydrogen peroxide following their oxidation.This is consistent with reports that show the continual formation of peroxide for several minutes following the end of electrolysis, as well as rotating ring disk electrochemical measurements that show a peroxide-oxidation current that is rotation speed (and consequently mass transport) dependent. 39If there was any surface-bound species beyond bi(carbonate), it was likely very short-lived and rapidly desorbed and thus its population is too low to be detected.Because HCO 4 À and C 2 O 6 2À both feature a distinct absorption band around 1,289 cm À1 that is noted in our spectra, we sought an alternative route to provide evidence for the identity of the primary CO -mediated mechanism prevailed.The other reactant, water, is assumed to be in excess.To this end, we performed a series of systematic experiments that measured both the FE and partial current density for peroxide as a function of CO 3 2À (Figure 5).Previously identified optimal conditions (2 V vs. RHE) were used, with SnO 2 catalysts.1 M KCl was used as a supporting electrolyte so electrolyte conductivity did not significantly vary across experiments.Across a wide concentration range, we observed a clear linear relationship in both FE and reaction rate and thus, we tentatively postulate that HCO 4 À is the key intermediate in our system.While this is not unambiguous proof, this assignment is consistent with our dataset.
From our experiments detailed previously, we believe that the depletion of CO 3 2À from the catalyst near-surface region is one of the limitations hindering the selectivity of the system.To see if we could, in part, circumvent this, we turned to the application of pulsed electrocatalysis.This technique entails the alternate applications of two or more different potentials as a function of time (Figure 6A), contrasting the constant-potential electrolysis that we have been using up to now.This technique has been successfully implemented in organic synthesis 40 and heterogeneous electrocatalysis, 41 though not yet in the context of CO 3

2À
-mediated peroxide synthesis.While this method can modulate reaction mechanisms by instituting a secondary reaction step 42 or alter the catalyst structure, 43 we sought to simply use the anodic voltage, V an , to carry out catalysis and the cathodic voltage, V ca (simply named for convenience as little current was passed), to enable the carbonate species to diffuse to the electrode surface and replenish the reactants available.In particular, we hypothesized that during nonreactive portions of the experiment, the replenishment of the near-surface region with reactants (CO 3

2À
) would yield higher selectivity.We also investigated several pulse durations (t an , t ca ) from 0.5 to 4.0 s, keeping the anodic and cathodic times equivalent.Modulating the pulse durations would determine the time allowed for near-surface concentrations of reactants and ions to equilibrate.The previously optimized V an was used as that is where the FE was the highest in constant potential measurements.V ca , in contrast, was modulated in an effort to minimize back-reactions while still allowing for re-equilibration of CO 3 2À concentrations.
We first saw that using a V ca of 1.5 V was detrimental to the system, no matter what pulse times were used (Figure 6B).We believe that the root cause of this was that at this potential, reactive species of HCO 4 À /C 2 O 6 2À or their derivatives may be reduced back to CO 3

2À
. When V ca was increased to 1.5 V vs. RHE, we saw an increase of FE.In this regime, there may still be some re-oxidation of intermediates but the increase stems from a more efficient replenishment of CO 3 2À to the electrode surface.Having a shorter pulse duration was more beneficial in this case as perhaps this minimized back-reduction of products while still enabling adequate diffusion.Finally, when V ca was set to 2.0 V vs. RHE, we obtained the best results, yielding an FE increase up to 27%.We believe that the re-reduction is likely minimized here.In addition, having a longer pulse duration of up to 4s for both the cathodic and anodic pulses that yield the highest peroxide selectivity may enable more reactants to diffuse to the surface.There was minimal current passed during the pauses in catalysis during the pulsing experiments and the anodic, catalytic current for pulsed and non-pulsed experiments did not vary significantly (Figure S17).Thus, we believe that changes in the species at the near-surface region shifted the catalysis from water oxidation to O 2 to the carbonate-mediated peroxide synthesis.There may be opportunities to push the selectivity for peroxide even higher through the use of electrolytes/solvents/conditions with even higher CO 3 2À solubility.

Limitations of the study
While we did not explore asymmetric pulses, sinusoidal-shaped potential vs. time applications, or multi-potential pulses as this is outside of the scope of the current work, these methods may yield even more substantial FE increases.In addition, we do not have direct proof that the difference in performance through pulsed electrolysis results from balancing mass transport and minimizing re-reduction of intermediates and this would be an interesting question to explore with time-resolved methods beyond our scope.The same goes for the infrared experiments, in which a new band not attributable to (bi)carbonate was detected though not unambiguously assigned, although we postulate it to be that from HCO 4 À from the first-order dependence of partial current density on CO 3 2À concentration.While the short-lived nature of this species renders it difficult to isolate and fully characterize, additional complementary techniques like electron paramagnetic resonance may prove beneficial in constructing a more comprehensive mechanistic picture.Experimental data such as performance across different catalysts/conditions and spectra from spectroelectrochemical experiments would further be strengthened through matching with predictions derived from computational modeling.We also demonstrated differences in activity with different catalysts and an open question remains as to why this is the case.While CO 3 2À oxidation is thought to be an outer-sphere reaction, differences in surface roughness, adsorption of chemical species (e.g., OER intermediates), double-layer composition, and more can potentially all influence the rate of the reaction and this is yet to be explored.
For the translation of practical systems, these findings imply that effective electrolyte flow near-surface mass transport is key toward maximizing selectivity toward peroxide production.Beyond this, there are certainly additional challenges that need to be solved like product separation, scale-up, and full system integration and we encourage researchers to consider how individual findings fit within progressing the field as a whole.

Concluding remarks
In this work, we investigated the carbonate-mediated oxidative electrosynthesis of peroxide from water with a combination of electrochemical and spectroscopic techniques.In our system, the results indicate the CO

Figure 1 .
Figure 1.General scheme Direct water oxidation reaction pathways (A) and our approach to help elucidate carbonate-mediated peroxide formation (B).

Figure 2 . 3 À.
Figure 2. System performance FE (A) and partial current density (B) for H 2 O 2 production for carbon paper with and without SnO 2 catalysts in a 2 M CO 3 2À electrolyte point to a beneficial role of the SnO 2 .Further, CO 3 2À is noted as most effective species for mediating the 2e À WOR under these conditions as evidenced from the FE (C) and partial current density (D) in comparison with CO 2 and HCO 3 À .Data are represented as mean G standard deviation.

Figure 3 .
Figure 3. Parameter studiesThe catalyst placed a role in the system performance as it affected the rate of carbonate oxidation (A, B).The pH was also critical to peroxide production and optimal around 12 (C, D).Finally, the role of other anions as mediators for peroxide production was ruled out as only CO 3 2À ions gave rise to substantial peroxide

Figure 4 .
Figure 4. Spectroelectrochemical studies IR spectra measured as a function of applied potential show the depletion of CO 3 2À and formation of HCO 3 À and HCO 4 À /C 2 O 6 2À (A), the latter is evidenced through its red-shift under 13 C-labeled measurements (B).As the potential was stepped from open circuit to 3 V vs. RHE.The rise of HCO 3 À mirrored the consumption of CO 3 2À but the band at 1,288 cm À1 attributed to HCO 4 À /C 2 O 6 2À reached its maximum much earlier and subsequently decreased (C).SERS spectra show the presence of CO 3 2À and HCO 3 À but surface-bound intermediates were not readily detected (D).

Figure 5 .
Figure 5. Concentration dependence The FE (A) and partial current density (B) for peroxide production both followed a linear relation with the concentration of CO 3 2À in the electrolyte, indicating firstorder kinetics.Data are represented as mean G standard deviation.

Figure 6 .
Figure 6.Pulsed electrolysis Schematic of pulsed-potential electrolysis (A) and summary of peroxide FE under different V ca and pulse durations (B).Data are represented as mean G standard deviation.
3 2À, and not HCO 3 À or CO 2 .is the dominant reactant leading to H 2 O 2 production.Following up with spectroscopic measurements, we provide evidence for the existence of a CO 3 2À , whose production may be limited by the depletion of near-surface CO 32À.Finally, we help to circumvent this issue through the application of pulsed electrolysis that enables the CO 3 2À reactants to diffusion back to the electrode surface.The insights from this work stand to help guide the development of practical H 2 O 2 electrosynthesis technologies.
STAR+METHODSDetailed methods are provided in the online version of this paper and include the following:d KEY RESOURCESTABLE d RESOURCE AVAILABILITY B Lead contact B Materials availability B Data and code availability d METHOD DETAILS